1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer-readable medium and, more particularly, to a technique of controlling image thinning processing.
2. Description of the Related Art
Today, an image processing apparatus such as a copying machine or printer which prints image data sent from an input apparatus such as a host computer or image scanner is widely used. In development by such an image processing apparatus, if a toner adhering amount is large, that is, if an amount of supplied toner is large, toner is scattered without completely adhering, thereby missing toner in a part where a dot density is high. It is known that the frequency of occurrence of such problem varies depending on environmental conditions such as a humidity and temperature in which the image processing apparatus is installed. Various types of media such as a paper sheet are used for printing, and a similar problem may occur depending on the type of medium. It is known that such problem tends to occur more frequently upstream in a medium conveyance direction.
When forming an image of a straight line in the main scanning direction, toner is scattered backward in the sub-scanning direction of the line recorded on a transfer material, thereby disturbing the image. Due to an abrupt increase in temperature when the transfer material passes through a fixing unit, water in the transfer material may evaporate. In this case, if the amount of supplied toner is large, there is no way for the water vapor of the transfer material to escape, which scatters the toner backward in the conveyance direction, thereby causing the above-described problem. In this specification, “the phenomenon of tailing-blur” indicates a phenomenon in which, for a straight line image in the main scanning direction, toner is scattered backward in the sub-scanning direction, thereby disturbing the image.
To avoid the phenomenon of tailing-blur, a method of decreasing a toner adhering amount has been conventionally adopted. In Japanese Patent Laid-Open No. 2004-314308, for example, since the phenomenon of tailing-blur tends to occur in an image region having specific features, an image region having such specific features is extracted, and the image data of the extracted image region undergoes thinning processing. This reduces a toner adhering amount while suppressing deterioration of the image quality.
In Japanese Patent Laid-Open No. 2004-314308, to extract an image region having specific features, an edge determination for a solid image region is made and whether the image region of a pixel of interest is to be processed is then determined on the assumption that input image data to be processed is a binary image. In Japanese Patent Laid-Open No. 2009-152766, to extract an image region, image data to be processed is extended to a multi-valued image by calculating a binarized threshold based on the adjacent pixel density of a pixel of interest, thereby improving the accuracy of edge determination.
Furthermore, it is known to switch execution of the above-described thinning processing depending on conditions. If image data read by an image scanner is input, it is difficult to extract an effective image region by making conventional edge determination due to unevenness of edges. It is, therefore, impossible to appropriately extract an image region to be thinned. To solve this problem, in Japanese Patent Laid-Open No. 2009-139664, if image data received from an external apparatus such as a host computer is input, image region extraction processing is executed by making edge determination, and the extracted region undergoes thinning processing. On the other hand, if image data read by an image scanner is input, a toner adhering amount is decreased by decreasing the developing property, for example, by reducing the direct-current component of a developing bias.
In thinning processing which improves the accuracy of extracting an image region by making edge determination, the toner scattering prevention effect improves but the processing time increases since the processing is complicated. In this specification, thinning processing in which the toner scattering prevention effect is high and the processing time is long will be referred to as “high image quality thinning processing”. To the contrary, thinning processing using conventional edge determination, in which the toner scattering prevention effect is low and the processing time is short, will be referred to as “low image quality thinning processing”. The high image quality thinning processing and the low image quality thinning processing will be explained in detail later in a description of embodiments.
Low image quality thinning processing can present a sufficient image quality for an image for which edge determination is easy. Since, however, whether edge determination is easy depends on the contents of an image, and cannot be determined prior to processing, high image quality thinning processing or low image quality thinning processing is selected and executed rather unconditionally in a conventional technique. If an image quality has priority, high image quality thinning processing is executed, which increases the processing time, thereby lowering the productivity of a printing operation as a whole.
If a software component executes thinning processing, the processing time changes depending on an operational environment. That is, in an environment where another processing (processing other than thinning processing) operates simultaneously with thinning processing, the processing time of the thinning processing may increase depending on the CPU activity ratio (a load on a CPU resource) of the other processing. In an environment where another processing simultaneously operates, therefore, the productivity of a printing operation as a whole lowers. Furthermore, since the processing time of high image quality thinning processing is inherently long, a decrease in productivity due to the influence of another processing is particularly large.